America is re-classifying information about its nuclear arsenal, much of it decades old, that has been in the public domain for several years.

Researchers at the National Security Archive, an independent library that belongs to The George Washington University, found that officials from the Pentagon and US Department of Energy have been trawling through reports that have been released to the public and deleting numbers of missiles, despite some of the statistics being decades old.

Dr William Burr, a senior analyst at the archive, said that figures released in the 1960s and 1970s, including some in annual reports published the Secretary of Defence, had been re-designated as secret.

During the Cold War, America regularly released details about its strategic nuclear weapons to advertise the scale of the deterrent facing the USSR.

Examples of redacted reports include a public presentation made by the then-Secretary of Defence, Melvin R. Laird, to the House Armed Services Committee in March 1971.

The corner of a chart showing the numbers of American intercontinental ballistic missiles (ICBMS), for instance "Minuteman: 1,000", has now been covered in black.

"It would be difficult to find better candidates for unjustifiable secrecy than decisions to classify the numbers of US strategic weapons," said Dr Burr.

"This problem, as well as the excessive secrecy for historical nuclear deployments, is unlikely to go away as long as security reviewers follow unrealistic guidelines."

The reclassification of the nuclear records forms part of a wider process that began in 1998 with the passing of the "Kyl-Lott Amendment", a piece of legislation designed to curtail an executive order signed by President Bill Clinton that ordered all US government agencies to release all information 25 years or older.

Since 1998, and increasingly under the Bush Administration, thousands of pages of historical records have been removed from public access, with particular attention to nuclear information and embarrassing incidents, according to historians and advocates for government transparency.

A separate report published by the National Security Archive in March, for instance, found that a faulty CIA intelligence estimate published 12 days before the Chinese army entered North Korea in 1950 was re-designated as secret in 2001, despite being written about by historians for years.

The National Security Archive estimated that the Department of Energy has spent $22 million re-classifying the nuclear information, reviewing around 200 million pages of released documents and removing 6,640 from the public record, at a cost of more than $3,300 per page.

Another document to be combed of nuclear statistics is a memorandum written by then Secretary of State Henry Kissinger in 1974.

In the note, addressed to President Gerald Ford, Dr Kissinger made reference to "200 older B-52 bombers" and 240 Trident missiles. First made public in 1999, a copy of the memo released by the Gerald Ford Library in May 2006 is now censored.

Bryan Wilkes, a spokesman for the National Nuclear Security Administration, part of the Energy Department, told The Washington Post that the Pentagon was behind the deleting of the missile numbers.

"There's no question that current classified nuclear weapons data was out there that we had to take back," he said.

"And in today's environment, where there is a great deal of concern about rogue nations or terrorist groups getting access to nuclear weapons, this makes a lot of sense."

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